So much this. Of all the characters in Avengers, a big reason why I loved Clint and Natasha the most was because they're normal. No enhancements, no super powers, just amazing agents with these awesome skills. Giving Natasha that agelessness takes away from her awesomeness, IMO. Even if the serum doesn't enhance her abilities at all, it means she's had decades worth of training to do that instead, which really turns me off. I would definitely prefer that whole subplot and her involvement with the Winter Soldier be left out. Having her be a super spy with knowledge of him would be enough.

MCU seems to borrow elements from many different universes, and this is a case that I really hope they don't follow 616.

I completely agree. In their drive to make characters more special, writers often end up making them less special. There is so much emphasis on making characters "relatable" to the audience now, but who is easier to relate to than a hero who is completely human? The fact that Natasha and Clint have no superhuman powers and yet they go out and fight alongside gods and monsters makes them the most heroic and interesting members of the team, in my opinion. Changing that, unnecessarily adding on enhancements to their abilities, detracts from their appeal instead of enhancing it.